This invention relates generally to pulse width modulation, and more specifically to converting either a pulse density modulated data stream or a pulse code modulated data stream to a pulse width modulated signal.
Pulse width modulation (PWM) is a suitable method to create a power signal with high efficiency. In particular, many high efficiency digital audio switching power amplifiers are based on PWM signaling. Digital audio inputs to these amplifiers are typically pulse code modulated (PCM). Direct translation from PCM to PWM to generate a uniformly sampled PWM (UPWM) signal is a nonlinear operation that results in a large amount of harmonic distortion. In contrast, naturally sampled PWM (NPWM) does not contain harmonic distortion. Naturally sampled PWM signals are easily generated in the analog domain by comparing an analog input signal to a sawtooth or triangular shaped ramp signal. The NPWM pulse edges are determined by the natural crosspoints between the input analog signal and the ramp signal. However, calculating the natural crosspoints for NPWM in the digital domain based on PCM input data can be computationally expensive.
Super Audio Compact Disc (SACD) is a new digital audio data format. The audio is digitized and stored in Pulse Density Modulation (PDM) format. It consists of an oversampled (64*Fs, where Fs is the initial sampling rate) one-bit PDM data stream. It is desirable to convert the SACD bit stream (or any PDM bit stream) to a Pulse Width Modulated (PWM) signal that can be used to drive a highly efficient switching digital audio amplifier. The SACD PDM bit stream can be directly used as a switching signal; however this approach does not readily allow for the implementation of any desired signal processing (i.e. volume control, equalization, and the like).
Pulse density modulated signals (such as SACD) are typically noise shaped in order to push the quantization noise out of the frequency band of interest. This results in a frequency spectrum that contains a large amount of out-of-band noise.
A very high-end switching digital audio amplifier for SACD input has been commercially introduced. However, in order to accommodate volume control it cannot directly amplify the SACD PDM signal. Instead, it must treat the PDM input signal as an analog signal;that can be attenuated as desired for volume control. This signal then feeds a seventh-order one-bit sigma delta ADC modulator that generates a new PDM signal for amplification in a switching amplifier. A big drawback of this system is that the signal does not stay in the digital domain. The digital input signal is converted to analog to allow signal processing in the analog domain, then converted to digital (PDM) to drive a switching amplifier. All the advantages of maintaining a digital signal line-up are lost. Additionally, the use of a PDM signal to drive a switching amplifier has some disadvantages compared to using a PWM signal. For example, PWM has a lower average switching frequency, which results in greater efficiency compared to PDM. Furthermore, the non-return-to-zero (NRZ) nature of the PDM signal can result in increased distortion compared to the return-to-zero PWM signal. One might consider processing the high-speed one-bit PDM signal in the digital domain (volume control, equalization, etc.) followed by a digital sigma delta modulator. However, processing at such a high bit rate would be extremely costly.
Many common approaches of SACD demodulation and amplification consist of decimating the high sample rate PDM to a low sample rate PCM, performing signal processing, performing digital to analog conversion, and amplifying in the analog domain. A significant drawback to this approach is that all advantages of high efficiency digital switching amplification are lost.
Therefore, a computationally efficient method is desirable to convert both PDM and PCM encoded input signals to a PWM switching waveform entirely in the digital domain in order to advantageously drive a switching digital power amplifier. This method should be tolerant of out-of-band noise as typically found with PDM signals.